Dqpsk/dpsk optical receiver with tunable optical filters

ABSTRACT

An optical receiver includes a first interferometer having a plurality of arms. The optical receiver further includes first tunable optical filters connected in series with the arms of the first interferometer, where each first tunable optical filter is tuned to filter a region of overlap in the optical frequency spectrum between adjacent optical channels.

BACKGROUND

Differential Phase Shift Keying (DPSK) and differential quadrature phaseshift keying (DQPSK) are modulation techniques used in optical datatransmission. Dense Wavelength Divisional Multiplexing (DWDM)transmission has been demonstrated over substantial optical fiberdistances at high rates (e.g., 100 Gb/s) using either DPSK or DQPSK.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a network according to an exemplaryimplementation;

FIG. 2A is a diagram of an exemplary configuration of the DQPSKtransmitter of FIG. 1;

FIG. 2B is a diagram of an exemplary configuration of a channel DQPSKunit of the DQPSK transmitter of FIG. 2A;

FIG. 3A is a diagram of an exemplary configuration of the DQPSK receiverof FIG. 1;

FIG. 3B is a diagram of an exemplary configuration of a channel DQPSKunit of the DQPSK receiver of FIG. 3A;

FIG. 4 illustrates an exemplary narrow optical pulse emitted by thepulse modulator of FIG. 2 in the time domain and the frequency domain;

FIG. 5 illustrates the frequency domain spectrum of FIG. 4 broadenedwith asymmetric blue and red shift due to fiber dispersion andnon-linearity;

FIG. 6 illustrates three exemplary optical channels, with wide channelspacing, such that there is very little overlap between the channelspectrums;

FIG. 7A illustrates three exemplary optical channels, with narrowchannel spacing, having substantial overlap between the channelspectrums;

FIG. 7B illustrates three exemplary optical channels having narrowchannel spacing and center frequencies that have drifted from nominal;

FIG. 7C illustrates the use of tunable optical filters in the DQPSKreceiver of FIG. 3A to filter regions of overlap between the channelspectrums of the narrow channel spacing of FIGS. 7B and 7C; and

FIGS. 8A and 8B are flowcharts of an exemplary process for tuning thetunable optical filters in the DQPSK receiver of FIG. 3A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description refers to the accompanying drawings.The same reference numbers in different drawings may identify the sameor similar elements. The following detailed description does not limitthe invention.

Exemplary embodiments described herein use tunable optical filters in anoptical receiver's interferometer(s) to reduce channel overlap (e.g.,cross talk) between adjacent optical channels in a DQPSK or DPSKmodulated system. When narrow channel spacing is employed, spectrumbroadening due to fiber dispersion and non-linearity can create regionsof substantial channel overlap that reduce system performance. Use oftunable optical filters in the DQPSK/DPSK receiver, as described herein,reduces this channel overlap and improves optical performance in anumber of ways that are further described below. Exemplary embodimentsdescribed herein have particular applicability to high speed (e.g., 100Gb/s) DQPSK/DPSK systems that employ narrow channel spacing.

FIG. 1 is a diagram of a network 100 according to an exemplaryimplementation. Network 100 may include a DQPSK transmitter 110, a DQPSKreceiver 120 and an optical network 130. As shown in FIG. 1, DQPSKtransmitter 110 may transmit DWDM DQPSK optical signals 140 to DQPSKreceiver 120 via optical network 130. DQPSK transmitter 110 may includean optical transmitter that uses DQPSK techniques for modulating opticalsignals to encode the signals with data and for transmitting themodulated optical signals across optical network 130 to DQPSK receiver120. DQPSK receiver 120 may include an optical receiver that uses DQPSKtechniques for receiving and demodulating optical signals received fromDQPSK transmitter 110 via optical network 130 and for extracting theoriginal data from the modulated optical signals. Optical network 130may include, for example, an optical fiber network which carries opticalsignals from one or more transmitters to one or more receivers.According to other exemplary embodiments (not shown), transmitter 110may include a DPSK transmitter, instead of a DQPSK transmitter, andreceiver 120 may include a DPSK receiver, instead of a DQPSK receiver.

FIG. 2A illustrates an exemplary configuration of DQPSK transmitter 110.As shown in FIG. 2A, DQPSK transmitter 110 may include one or morechannel DQPSK units 200-1 through 200-N (individually and genericallyreferred to herein as a “channel DQPSK unit 200-x”) and a multiplexer205. Each channel DQPSK unit 200-x may include components fortransmitting data via optical pulses over a specific optical channel.Each DQPSK unit 200-x, thus, may transmit data over a different opticalchannel (e.g., at a different optical wavelength). Multiplexer 205 mayinclude a wavelength division multiplexing device that multiplexesdifferent channels of optical signals into an optical fiber. Multiplexer205 may include, for example, an optical add-drop multiplexer (OADM).Multiplexer 205 may multiplex optical pulses of the different channelstransmitted by channel DQPSK units 200-1 through 200-N to produce aDQPSK signal out.

FIG. 2B is a diagram of an exemplary configuration of a channel DQPSKunit 200-x. As depicted in the exemplary configuration of FIG. 2B, DQPSKunit 200-x may include a continuous wave (CW) laser 210, a pulsemodulator 215, a clock driver 220, a data divider and encoder 225, datadrivers 230 and 235, and an interferometer 240.

CW laser 210 may include a laser source that produces optical CW carriersignals at a given wavelength (e.g., a single channel). Pulse modulator215 may pulse modulate the carrier signal from CW laser 210, based onclock signals received from clock driver 220, to generate opticalpulses. Data divider and encoder 225 may include circuitry for dividingthe data to be transmitted (e.g., data at 100 Gb/s) into two streams ofdata (e.g., 50 Gb/s for each data stream) and may encode each stream ofdata for providing to data drivers 230 and 235. Data driver 230 mayoutput signals, based on encoded data received from data divider andencoder 225, to one arm of interferometer 240. Data driver 235 mayoutput signals, based on encoded data received from data divider andencoder 225, to another arm of interferometer 240.

Interferometer 240 may include an optical splitter 245, a first phasemodulator 250, a second phase modulator 255, a phase shifter 260 and anoptical coupler 265. Optical splitter 245 may split an incoming opticalpulse into two substantially identical pulses and provide each of theoptical pulses to a different arm of interferometer 240. One of theoptical pulses from splitter 245 may be provided to phase modulator 250in a first arm of interferometer 240, and the other to phase modulator255 in a second arm of interferometer 240. Phase modulator 250 maymodulate the phase of the received optical pulse based on signalsreceived from data driver 230. Phase modulator 255 may modulate thephase of the received carrier wave optical pulse based on signalsreceived from data driver 235. Phase modulator 255 may provide the phasemodulated optical pulse to phase shifter 260, which may then phase shiftthe modulated carrier wave signal a specified amount. In oneimplementation, phase shifter 260 may include a π/2 phase shifter thatinduces 90 degrees of phase shift in the phase modulated optical pulse.The phase shifted optical pulse output from phase shifter 260 may beprovided to optical coupler 265. Optical coupler 265 may include, forexample, a 50/50 coupler and may couple the phase modulated opticalpulses from phase modulator 250 and from phase modulator 255 to providean output signal that can be provided to MUX 205 for multiplexing.

FIG. 3A illustrates an exemplary configuration of DQPSK receiver 120. Asshown in FIG. 3A, DQPSK receiver 120 may include a demultiplexer 300 andone or more channel DQPSK units 305-1 through 305-N (individually andgenerically referred to herein as a “channel DQPSK unit 305-x”).Demultiplexer 300 may include a wavelength division demultiplexingdevice that demultiplexes different channels of optical signals out ofan optical fiber for provision to different ones of DQPSK units 305-1through 305-N. For example, demultiplexer 300 receives the DQPSK signalin, which includes optical signals in multiple channels (e.g., channels1 through N) and demultiplexes signals from each channel to a respectivechannel DQPSK unit 305-x. Each of channel DQPSK units 305-1 through305-N demodulates the received optical signals in a respective opticalchannel to retrieve the data encoded in the optical signals.

FIG. 3B is a diagram that illustrates an exemplary configuration of achannel DQPSK unit 305-x. As depicted in FIG. 3B, channel DQPSK unit305-x may include an optical splitter 310, a first interferometer 315, asecond interferometer 320 and a data combiner 325. Optical splitter 310may split each DQPSK optical pulse received from demultiplexer 300 intotwo DQPSK optical pulses and may provide a first one of the pulses tointerferometer 315 and a second one of the pulses to interferometer 320.

Interferometer 315 may include an optical splitter 327 that connects toa first arm 329 and a second arm 330 of interferometer 315. Opticalsplitter 327 may split an incoming optical pulse into two substantiallyidentical pulses and provide each of the optical pulses to a differentarm of interferometer 315 (i.e., one pulse to arm 329 and another pulseto arm 330). Arm 329 may include a tunable optical filter 332 and aphotodetector 334. Arm 329 may optionally also include a tunableelectrical filter 335 connected to the output of photodetector 334. Arm330 may include a tunable optical filter 337 and a photodetector 339.Arm 330 may optionally also include a tunable electrical filter 340connected to the output of photodetector 339. Tunable optical filters332 and 337 may be appropriately tuned to filter channel overlapregions, as further described below, and reduce channel cross-talk.Optional tunable electrical filters 335 and 340 may also be tuned, inconjunction with the tuning of optical filters 332 and 337, to reduceexcessive frequency domain overlapping and thereby reduce frequencyoverlapping noise. Tunable optical filters 332 and 337 may includetunable optical band reject filters, such as, for example, narrowbandoptical tunable thin filter filters. Photodetectors 334 and 339 detecteach received optical pulse and convert the optical pulse into an outputelectrical signal. Photodetectors 334 and 339 may include positiveintrinsic negative (PIN) diodes, avalanche photo detectors (APDs),and/or photo-multiplier tubes (PMTs). The electrical signal output fromphotodetectors 334 and 339 may be input into a comparator 355 (or intofilters 335 and 340 and then into comparator 345). Arm 330 may induce adelay (e.g., a one pulse delay) in received optical pulses relative tooptical pulses received at arm 329 of interferometer 315. Thus, for anygiven pulse detected at photodetector 334 of arm 329, the previousoptical pulse will be detected at photodetector 339 enabling comparator355 to subtract the electrical signals from one another to determine thephase difference between sequential optical pulses.

Interferometer 320 may include an optical splitter 350 that connects toa first arm 352 and a second arm 354 of interferometer 320. Opticalsplitter 350 may split an incoming optical pulse into two substantiallyidentical pulses and provide each of the optical pulses to a differentarm of interferometer 320 (i.e., one pulse to arm 352 and another pulseto arm 354). Arm 352 may include a tunable optical filter 355 and aphotodetector 357. Arm 352 may optionally also include a tunableelectrical filter 359 connected to the output of photodetector 357. Arm354 may include a tunable optical filter 360 and a photodetector 362.Arm 354 may optionally also include a tunable electric filter 364connected to the output of photodetector 362. Tunable optical filters355 and 360 may be appropriately tuned to filter channel overlapregions, as further described below, and reduce channel cross-talk.Optional tunable electrical filters 359 and 364 may also be tuned, inconjunction with the tuning of optical filters 355 and 360, to reduceexcessive frequency domain overlapping and thereby reduce frequencyoverlapping noise. Tunable optical filters 355 and 360 may includetunable optical band reject filters, such as, for example, narrowbandoptical tunable thin filter filters. Photodetectors 357 and 362 maydetect each received optical pulse and convert the optical pulse into anoutput electrical signal. Photodetectors 357 and 362 may includepositive intrinsic negative (PIN) diodes, avalanche photo detectors(APDs), and/or photo-multiplier tubes (PMTs). The electrical signaloutput from photodetectors 357 and 362 may be input into a comparator365 (or into filters 359 and 364 and then into comparator 365). Arm 354may induce a delay (e.g., a one pulse delay) in received optical pulsesrelative to optical pulses received at arm 352 of interferometer 320.Thus, for any given pulse detected at photodetector 357 of arm 352, theprevious optical pulse will be detected at photodetector 362 enablingcomparator 365 to subtract the electrical signals from one another todetermine the phase difference between sequential optical pulses.

Data combiner 325 may receive the phase difference signals fromcomparators 345 and 365 and may determine the data values (e.g., 00, 01,10, 11) that correspond to those phase difference signals.

FIG. 4 depicts an example of an optical pulse 400, in the time domain,which may be emitted by pulse modulator 215 of a channel DQPSK unit200-x of DQPSK transmitter 110. As further shown in FIG. 4, the timedomain optical pulse 400 may have a corresponding optical spectrum 410in the frequency domain. As additionally depicted in FIG. 5, the opticalspectrum 410 associated with optical pulse 400 can undergo asymmetricbroadening, with asymmetric blue and red shifts, as optical pulse 400traverses optical network 130 from transmitter 110 to receiver 120 dueto fiber dispersion and non-linearity.

FIG. 6 further depicts the use of multiple channels (e.g., multipledifferent optical wavelengths) for transmitting data between transmitter110 and receiver 120 via optical network 130. In the example of FIG. 6,the channel spacing is set relatively far apart, resulting in verylittle channel spectrum and phase overlap. As shown in FIG. 6, amulti-channel frequency spectrum 600 may include channel 1 610, channel2 620 and channel 3 630, with each having respective center frequenciesf_(c1), f_(c2) and f_(c3). As further shown in FIG. 6, overlap regionsexist (e.g., overlap region 640 between channel 1 610 and channel 2 620and overlap region 650 between channel 2 620 and channel 3 630) thathave minimal overlap between the spectrums of the adjacent channels.

In the case where narrow channel spacing is employed, such as in theexample shown in FIG. 7A, a relatively large amount of undesirablechannel spectrum and phase overlap occurs. As depicted in FIG. 7A, amulti-channel frequency spectrum 700 may include channel 1 710, channel2 720 and channel 3 730, with each having respective center frequenciesf_(c1), f_(c2) and f_(c3). As further shown in FIG. 7A, overlap regionsexist (e.g., overlap region 740 between channel 1 710 and channel 2 720,and overlap region 750 between channel 2 720 and channel 3 730) thathave substantial overlap between the spectrums of the adjacent channels(e.g., substantial cross-talk between channels). Overlap regions betweenadjacent channels that have been asymmetrically broadened may havesignificantly increased channel overlap. For example, overlap region 740between channel 1 710 and channel 2 720 includes a large channel overlapdue to asymmetric broadening of the channel spectrum.

FIG. 7B illustrates additional channel spectrum and phase overlap thatmay occur due to center frequency drift associated with each channel.The center frequency drift may occur as a result of frequency error inCW laser 210 for a respective channel, or may occur due to frequencydrift that may be induced by multiplexer 205 of DQPSK transmitter 110 ordemultiplexer 300 of DQPSK receiver 120. As shown in FIG. 7B, eachchannel may have a drift in the center frequency from a nominal centerfrequency. For example, channel 1 710 may have an actual centerfrequency “f_(c1) actual” that represents a frequency drift Δf_(c1) fromthe nominal center frequency “f_(c1) nominal.” As another example,channel 2 720 may have an actual center frequency “f_(c2) actual” thatrepresents a frequency drift Δf_(c2) from the nominal center frequency“f_(c2) nominal.” As an additional example, channel 3 730 may have anactual center frequency “f_(c3) actual” that represents a frequencydrift Δf_(c3) from the nominal center frequency “f_(c3) nominal.”

To reduce optical cross-talk in the optical spectrum 700 depicted inFIG. 7A or 7B, tunable optical filters may be used in receiver 120. Forexample, tunable optical filters 332, 337, 355 and 360 of a channelDQPSK unit 305-x of receiver 120 may be used to decrease the cross-talk(e.g., filter the cross-talk) in the overlap regions 740 and 750 ofspectrum 700 when the channels are narrowly spaced. As shown in FIG. 7C,tunable optical filters 332, 337, 355 and 360 of receiver 120 may act tofilter regions 760 and 770 to eliminate or reduce channel overlap. Useof tunable optical filters in receiver 120, as described herein,improves optical performance particularly when employing narrow channelspacing (e.g., reduced adjacent channel cross-talk), improves opticalpulse asymmetrical broadening in the frequency domain, improvespolarization mode dispersion (PMD) and residual dispersion correctionand tolerances, improves the margins for the DWDM tunable laser used inthe DQPSK transmitter (e.g., increased tolerance in the laser centralfrequency drift from International Telecommunication Union (ITU) grid),relaxes the specifications in the design of the DQPSK transmitter, andmay improve Optical Signal to Noise Ratio (OSNR) by, for example, 1-3dB. The use of tunable optical filters in the optical receiver, asdescribed herein, has particular applicability to narrow channel spacingDense Wavelength Division Multiplexing (DWDM) systems as compared towide channel spacing or coarse wave length division multiplexing (CWDM)systems. Optimum settings of each of tunable optical filters 332, 337,355 and 360 of receiver 120 may be found through performance testing.Settings of each of the tunable filters may be determined that providethe desired improvements in performance at receiver 120.

FIGS. 8A and 8B are flow diagrams of an exemplary process for tuning thetunable optical filters in receiver 120 to provide desired receiverperformance improvements (e.g., reduction of channel crosstalk). Theexemplary process of FIGS. 8A and 8B may be manually performed or may beperformed by an automated performance testing system. The flow diagramsof FIGS. 8A and 8B refer to an “optical filter A” and an “optical filterB.” “Optical filter A” may correspond to a tunable optical filter in afirst arm of an interferometer of a channel DQPSK unit 305-x, and“optical filter B” may correspond to a tunable optical filter in asecond arm of an interferometer of a channel DQPSK unit 305-x. Forexample, “optical filter A” may correspond to tunable optical filter 332in arm 329 of interferometer 315 and “optical filter B” may correspondto tunable optical filter 337 in arm 330 of interferometer 315. Asanother example, “optical filter A” may correspond to tunable opticalfilter 355 in arm 352 of interferometer 315 and “optical filter B” maycorrespond to tunable optical filter 360 in arm 354 of interferometer315.

The exemplary process may begin with the tuning of optical filter A andoptical filter B to a same center frequency (block 800). The same centerfrequency may correspond to the nominal center frequency for the channelassociated with the respective channel DQPSK unit 305-x. Optical filterA may then be tuned to selected positions left and right of the centerfrequency, leaving optical filter B fixed at the center frequency, and asignal bit error rate (BER) and/or Q value may be measured and recordedat each filter position (block 805). Optical filter B may then be tunedto selected positions left and right of the center frequency, leavingoptical filter A fixed at the center frequency, and a signal BER and/orQ value may be measured and recorded at each filter position (block810).

Optical filter A may be tuned to an offset from the center frequencythat is less than the center frequency and optical filter B may be tunedto an offset from the center frequency that is greater than the centerfrequency (block 815). Optical filters A and B may then be tunedsimultaneously, from their respective offsets, towards the centerfrequency and the signal BER and/or Q value may be measured and recordedat each filter position (block 820).

Optical filter A and optical filter B may be tuned to a same centerfrequency (block 825). Optical filter A and optical filter B may then betuned simultaneously away from the center frequency and the signal BERand/or Q value may be measured and recorded at each filter position(block 830). In this block, optical filter A and optical filter B may besimultaneously tuned away from the center frequency in oppositedirections. Thus, optical filter A may be tuned towards decreasingfrequencies while optical filter B may be tuned towards increasingfrequencies, and then optical filter A may be tuned towards increasingfrequencies while optical filter B may be tuned towards decreasingfrequencies.

Optical filter A and optical filter B may be tuned to an offset from thecenter frequency that is less than the center frequency (block 835).Optical filter A and optical filter B may then be simultaneously tunedtowards increasing frequencies and the signal BER and/or Q value may bemeasured and recorded at selected filter positions (block 840).

Optical filter A and optical filter B may be tuned to an offset from thecenter frequency that is greater than the center frequency (block 845).Optical filter A and optical filter B may then be simultaneously tunedtowards decreasing frequencies and the signal BER and/or Q value may bemeasured and recorded at selected filter positions (block 850).

The filter positions of optical filters A and B that maximize BER and/orQ value performance may be determined based on previous filter tuningsand a comparison of recorded BER and/or Q values (block 855). RecordedBER and/or Q values associated with each of the filter tuning blocks(e.g., blocks 805, 810, 820, 830, 840 and 850 of FIGS. 8A and 8B) may becompared with one another to deduce the optimum filter positions ofoptical filters A and B that maximize BER and/or Q value performance.For example, the recorded BER values associated with each of the filtertuning blocks of FIGS. 8A and 8B may be compared with one another todeduce the optimum filter positions of optical filters A and B thatproduce the smallest BER.

The blocks of the flow diagram of FIGS. 8A and 8B have been describedwith respect to tuning optical filters A and B, which may correspond totunable optical filters 332 and 337 or 355 and 360 of FIG. 3B. Inadditional exemplary embodiments, tunable electrical filters 335 and 340or 359 and 364 may be tuned in conjunction with the tuning of opticalfilters 332 and 337 or 355 and 360 to maximize BER and/or Q valueperformance.

In the preceding specification, various preferred embodiments have beendescribed with reference to the accompanying drawings. It will, however,be evident that various modifications and changes may be made thereto,and additional embodiments may be implemented, without departing fromthe broader scope of the invention as set forth in the claims thatfollow. The specification and drawings are accordingly to be regarded inan illustrative rather than restrictive sense. Modifications andvariations are possible in light of the specification, or may beacquired from practice of the invention. For example, embodiments havebeen described herein with respect to a DQPSK receiver. However, tunableoptical filters (and tunable electrical filters) may be similarlyemployed in the DPSK receiver of a DPSK system for similar improvementsin optical performance, as described herein.

It will be apparent that embodiments, as described above, may beimplemented in many different forms of software, firmware, and hardwarein the implementations illustrated in the figures. The actual softwarecode or specialized control hardware used to implement embodiments isnot limiting of the invention. Thus, the operation and behavior of theembodiments have been described without reference to the specificsoftware code, it being understood that software and control hardwaremay be designed based on the description herein.

No element, act, or instruction used in the present application shouldbe construed as critical or essential to the invention unless explicitlydescribed as such. Also, as used herein, the article “a” is intended toinclude one or more items. Where only one item is intended, the term“one” or similar language is used. Further, the phrase “based on” isintended to mean “based, at least in part, on” unless explicitly statedotherwise.

1-25. (canceled)
 26. A method comprising: tuning an optical filter in anarm of an interferometer; measuring at least one of bit error rates(BERs) or Q-values associated with the interferometer during the tuningof the optical filter; and maximizing, based on the measuring, at leastone of BER performance or Q-value performance of the interferometer. 27.The method of claim 26, where tuning the optical filter includes: tuningthe optical filter to a first plurality of filter positions, where oneof the first plurality of filter positions is left of a center frequencyand another one of the first plurality of filter positions is right ofthe center frequency, and where another optical filter in another arm ofthe interferometer is positioned at the center frequency during thetuning of the optical filter.
 28. The method of claim 27, where tuningthe optical filter further includes: tuning the other optical filter toa second plurality of filter positions, where one of the secondplurality of filter positions is left of the center frequency andanother one of the second plurality of filter positions is right of thecenter frequency, and where the optical filter is positioned at thecenter frequency during the tuning of the other optical filter.
 29. Themethod of claim 26, where tuning the optical filter includes:simultaneously tuning the optical filter and another optical filter, theother optical filter being positioned in another arm of theinterferometer, where the optical filter is tuned from a first offsetfrom a center frequency towards the center frequency, and the otheroptical filter is tuned from a second offset from the center frequencytowards the center frequency, and where the first offset is less thanthe center frequency and the second offset is greater than the centerfrequency.
 30. The method of claim 29, where tuning the optical filterfurther includes: simultaneously tuning the optical filter and the otheroptical filter, where the optical filter is tuned in a first directionaway from the center frequency and the other optical filter is tuned ina second direction away from the center frequency, where the seconddirection is opposite to the first direction; and simultaneously tuning,from the center frequency, the optical filter and the other opticalfilter, where the optical filter is tuned in the second direction andthe other optical filter is tuned in the first direction.
 31. The methodof claim 26, where tuning the optical filter includes: tuning, from afirst offset from a center frequency, and towards the center frequency,the optical filter, where the first offset is less than the centerfrequency.
 32. The method of claim 26, where maximizing the at least oneof the BER performance or the Q-value performance includes: determining,based on the measured at least one of BERs or the Q-values, a filterposition, where the determined filter position produces a smallest BERfor the interferometer.
 33. An optical receiver comprising: aninterferometer; and a tunable filter that is associated with theinterferometer, where the tunable filter is to reduce channel overlapbetween adjacent optical channels.
 34. The optical receiver of claim 33,further comprising: another tunable filter that is associated with thetunable filter, where the other tunable filter is to reduce frequencydomain overlap between the adjacent optical channels.
 35. The opticalreceiver of claim 33, where the tunable filter is further to: correctfor asymmetrical broadening of optical spectrums associated with one ormore of the adjacent optical channels.
 36. The optical receiver of claim33, where the tunable filter is further to: correct for a centerfrequency drift associated with one or more of the adjacent opticalchannels.
 37. The optical receiver of claim 33, where the opticalreceiver comprises: a differential quadrature phase shift keying (DQPSK)or a differential phase shift keying (DPSK) optical receiver.
 38. Theoptical receiver of claim 33, where the adjacent optical channelscomprise a plurality of optical channels having a narrow channelspacing.
 39. The optical receiver of claim 33, where the tunable filteris further to: filter out adjacent channel cross talk.
 40. A systemcomprising: a device to: filter a region of overlap between adjacentoptical channels.
 41. The system claim 40, where the device includes: atunable filter.
 42. The system of claim 40, where the device includes:an interferometer.
 43. The system of claim 42, where the device furtherincludes: a tunable filter connected to the interferometer.
 44. Thesystem of claim 43, where the device includes: an optical splitterconnected to the interferometer.
 45. The system of claim 40, where, whenfiltering the region of overlap, the device is further to: reducefrequency domain overlap between the adjacent optical channels.